1. Field of the Invention
The field of art to which this invention pertains is hydrocarbon separation. More specifically, this invention relates to a process for separating olefins from a hydrocarbon feed mixture containing olefins and saturates which process employs a zeolitic adsorbent.
2. Description of the Prior Art
The treating of zeolites with certain substances to modify certain of these properties when used in various separation processes has been recognized by the prior art. U.S. Pat. No. 3,106,593 for instance teaches the use of nitrogen-containing bases such as ammonia and various amines to neutralize surface acidity of certain zeolites used to separate olefins; U.S. Pat. No. 3,698,157 teaches that contacting ZSM-5 and ZSM-8 zeolites with organic-radical substituted silanes improves the selectivity of the zeolites for para-xylene with respect to the other xylenes when used in a xylene separation process; U.S. Pat. No. 3,855,333 teaches that contacting X or Y zeolites with certain alcohols improves the selectivity of the zeolite for the para-isomer with respect to other isomers when used in a para-isomer separation process. More specifically, the prior art has also recognized that treating certain zeolites with caustic solutions will modify particular zeolite properties and produce certain results depending on the type of zeolite and the treating conditions employed.
U.S. Pat. No. 3,326,797, for example, discloses treating high-silica zeolites, having silica to alumina mole ratios between about 6 and 12 (for example, mordenite) with caustic solutions for the purpose of removing a certain percentage of structural silica from the zeolites. The caustic treatment of such a high-silica zeolite, at conditions to preferably retain a final SiO.sub.2 /Al.sub.2 O.sub.3 ratio greater than about 5.5, is found to increase the adsorptive capacity of the zeolite and to increase its catalytic activity particularly when used as a catalyst in such acid-catalyzed reactions such as cracking. The caustic treating of this patent is concerned with etching or leaching of silica from the particular zeolite structure to achieve these characteristics and does not disclose or suggest the addition of alkali metal cations to the zeolite structure during the treating process.
U.S. Pat. No. 3,717,572 discloses treating A zeolites with nitrogen-free base solutions to modify "surface acidity" of the zeolite when used in a process for separating straight-chain olefins from branch-chain olefins. In contrast to the high-silica zeolites of U.S. Pat. No. 3,326,797, the type A zeolite is characterized by a rather low silica to alumina mole ratio, as taught in U.S. Pat. No. 2,882,243, within the range of 1.85.+-.0.5. The treatment of such zeolites appears to be merely an acid-base reaction to neutralize surface acidity; the base appears not critical and can be generally bases whose cation is an alkaline metal, for instance, soda, potash, and lithium hydroxide. Treatment conditions are such that the concentration of the base solution after impregnation is about the same as the fresh base solution. The result desired and that produced with the particular zeolite is only neutralization of acidity; indeed, the preferred solvent for the base solution is an organic medium (in particular methanol) since in such cases neutralization is obtained while maintaining a practically unchanged adsorptive capacity of the sieve. Neutralization by aqueous base solutions actually lowers this capacity.
Thus the prior art has recognized that similar base treatments of different zeolites can produce different results. Such is not surprising since the zeolites vary considerably in such chemical and physical properties as composition, silica to alumina mole ratio, symmetry, cell dimensions, structure, density, pore diameter, and surface area.
The process of our invention employs an adsorbent comprising sodium type X or sodium type Y zeolite prepared by a method which includes a treatment step of a precursor mass comprising type X or Y zeolite with an aqueous caustic solution. We have found that the treatment of such precursor mass at particular treatment conditions will produce an adsorbent having increased capacity for olefins and decreased catalytic activity when used in a process for separating olefins from paraffins. We have found that catalytic activity of the finished adsorbent decreases in proportion to the amount of sodium cations added to the zeolite by the caustic treatment. Specifically, we have found that a sodium content, expressed as the ratio Na.sub.2 O/Al.sub.2 O.sub.3, above about 0.7 is required to produce an adsorbent having the desired properties. The sodium cation added by the ion-exchange apparently replaces acid sites within the zeolite that catalyze isomerization and polymerization reaction. The removal of a small amount of silica and alumina from the precursor mass results in improved capacity of the adsorbent for olefins.
Employing the adsorbent so produced in an olefin separation process results in an improved olefin separation process because less adsorbent is required due to the adsorbent's increased capacity and because the adsorbent has a larger effective on-stream life due to its reduced catalytic activity.